1. Field of the Invention
The present invention relates to a process for forming stable silicides, and more specifically, to a process of excimer laser crystallization of amorphous silicon deposited on top of a layer of unstable silicide to form thermally stable silicide having low electrical resistance.
2. Description of the Related Art
With the decreased size of semiconductor devices, the sheet resistivity of the electrically-conducting structures, such as the gates of MOS transistors, emitters of bipolar transistors, local interconnect regions of MOS and bipolar transistors, and interconnect lines connecting devices together, is beginning to limit the speed of operation.
One well-known technique for reducing sheet resistivity is to form a layer of metal silicide over the electrically conducting structures. TiSi.sub.2 and CoSi.sub.2 are the most extensively utilized metal silicides, although other refractory metals such as platinum, palladium, and nickel may also form silicides. Silicided structures provide the lower resistivity of a metal silicide, together with the well-known attributes of silicon.
FIGS. 1A-1E show the conventional formation of a layer of thermally-stable silicide. FIG. 1A shows silicon 2, adjacent to non-silicon 3. Silicon 2 is typically of single crystal form, but may also be polycrystalline. Non-silicon 3 may be oxide, metal, or some other non-silicon feature typically present in modern semiconductor devices.
FIG. 1B shows the deposition of metal layer 4 on top of silicon 2 and non-silicon 3. Metal layer 4 may be composed of a variety of refractory metals, with cobalt or titanium being preferred.
FIG. 1C shows the first conventional low temperature annealing step to form thermally unstable silicide 6 having metal-rich surface region 6a, on top of silicon 2. Non-silicon 3 does not react with metal 4 to form unstable silicide 6 due to an absence of silicon to serve as a template for silicide formation.
During this first, low-temperature annealing step, suicides generally form on top of silicon 2 in a variety of thermally unstable phases that display relatively high sheet resistance. The first annealing step for CoSi.sub.2 takes place at a temperature of about 500.degree. C. For TiSi.sub.2, at temperatures below approximately 650.degree. C. a C-49 phase is initially formed which has a relatively small grain size (.apprxeq.1 .mu.m) and high sheet resistance.
FIG. 1D shows removal of unreacted metal layer 4 using conventional wet etching techniques. FIG. 1E shows the conventional second annealing step, wherein the silicon 2 and unstable silicide 6 are subjected to substantially higher temperatures than in the first annealing step. During this second annealing step, thermally unstable silicide phase 6 having metal-rich surface layer 6a is transformed into a thermally stable polycrystalline silicide phase possessing lower sheet resistance. For example during the formation of TiSi.sub.2, the second annealing step promotes formation of the C-54 phase TiSi.sub.2, which has a larger grain size (&lt;1 .mu.m) and much lower sheet resistance than the C-49 phase.
In order to form the thermally stable C-49 phase of TiSi.sub.2, the second anneal must take place at a temperature of about 850.degree. C. To form thermally stable CoSi.sub.2, the second annealing temperature must take place at about 650.degree. C.
There are several major problems associated with forming silicides utilizing the conventional two-step annealing method described above. First, the high temperatures necessary for the second rapid thermal processing annealing step shown in FIG. 1D create device reliability problems. Specifically, heating the entire silicon region to temperatures above 650.degree. C. can cause unwanted diffusion of conductivity-altering dopant already precisely introduced into the silicon. Moreover, high temperatures during the second annealing step can cause unwanted lateral outgrowth of silicon oxide structures that have already been formed, for example narrow spacers protecting the edges of gate structures present on MOS transistor devices.
Therefore, it is desirable to utilize a process of forming silicides that does not require subjecting all of the silicon at once to the high temperatures associated with a conventional second annealing step.
A second problem associated with forming silicides in the conventional manner is that disorderly silicide exhibiting relatively high electrical resistance tends to be produced. This is disadvantageous insofar as polycrystalline silicide generally possesses a sheet resistance that is two to three times greater than that of highly ordered, single crystal silicide.
Sigmon, Materials Research Society Bulletin, 1981, p. 511, summarizes efforts by researchers to form silicides by methods other than rapid thermal processing annealing. One proposed alternative is the use of ruby or Nd:YAG pulsed lasers to produce an interdiffusion between the silicon substrate and the deposited metal film followed by reaction to form the silicide. However, the melted silicon produced by long wavelength lasers (.lambda..gtoreq.0.7 .mu.m) such as ruby or Nd:YAG typically has quenching characteristics that result in the formation of a variety of silicide phases, some of which have unfavorable electrical properties. This is because the relatively long wavelengths of radiation produced by Nd:YAG and ruby lasers permits them to penetrate deeply into the silicon, producing a temperature gradient that is highest at the surface and declines gradually at greater depths. Because the formation of silicide phases is temperature dependent, the resulting temperature gradient yields a variety of silicide phases.
Therefore, it is desirable to utilize a process of laser annealing that promotes formation of a single, thermally-stable silicide phase possessing low sheet resistance.
Sigmon also discusses application of radiation from a continuous wave (CW) Argon-Ion laser. Use of a CW Argon-ion laser to cause the reaction to occur in the solid or liquid state, resulting in production of only one single silicide phase. Unfortunately, the total output power of a typical commercial CW Argon-ion laser is less than 50 W, with output power at UV frequencies of less than 5 W. Thus in using a CW Argon-ion laser, the cross-sectional area of the applied beam must be extremely narrow in order to achieve energy densities sufficient to anneal the silicide. Because of the difficulty in achieving full coverage over even small silicon areas with a beam possessing such a small cross-section, CW Argon-ion lasers cannot feasibly be utilized in CMOS applications due to the low throughput and difficulties with process control.
Therefore, it is desirable to form stable silicides by a process of laser annealing that utilizes a laser having sufficient energy density and cross-sectional beam area to promote the formation of stable silicides in a process having high throughput and substantial process efficiency.